Ovarian cancer is the fifth leading cause of cancer death among U.S. women, primarily from advanced high-grade serous ovarian adenocarcinoma (HGS-OvCa). Inherited mutation of BRCA1 and BRCA2, which function in homologous recombination repair (HRR), leads to a substantial increase in ovarian cancer risk. The Cancer Genome Atlas (TCGA) project recently identified unusually high genomic instability as a prominent feature of HGS-OvCa tumors and estimated that ~50% have HRR defects. Since this frequency significantly exceeds that of BRCA mutations, it is likely that changes in HRR genes not known to be associated with inherited predisposition syndromes are important in the etiology of sporadic ovarian cancer. This project will explore a novel role for the highly multifunctional DNA repair protein XPG as a driver in ovarian carcinogenesis. Surprising recent findings have established that XPG functions in HRR through direct interaction with several members of the HRR pathway, notably including both BRCA proteins as well as the RAD51 recombinase, and that loss of XPG has significant consequences for impairment of BRCA1 and BRCA2 functions. Knockdown of XPG leads to reduced HRR, inability to restart stalled replication forks, and a dramatic increase in genomic instability. Conversely, the enzymatic function of XPG is required in nucleotide excision repair (NER) to remove adducts formed by many chemotherapeutic agents, and acquired resistance to first-line therapies such as cisplatin and carboplatin has been linked to over-expression of NER proteins including XPG. Importantly for development of new therapeutic approaches, tumors with diminished HRR due to low expression of XPG may be particularly sensitive to PARP inhibitors. The hypothesis to be tested by the proposed exploratory studies is that XPG is a novel tumor suppressor for ovarian carcinogenesis that its loss or down- regulation is a previously unidentified factor in the etiology of ovarian cancer, and that understanding XPG protein regulation and activity in ovarian cancer cells has direct therapeutic implications. This hypothesis will be tested through two specific aims. (1) The effect of XPG loss on genomic instability and cellular transformation in immortalized, non-tumorigenic ovarian surface epithelial (OSE) cells and fallopian tube secretory epithelial cells (FTSEC), which have each been proposed to be the precursor cell types for ovarian carcinogenesis, will be determined. (2) XPG protein amounts, function, and regulation will be examined in a panel of ovarian cancer cell lines that have been shown to most closely resemble primary HGS-OvCa tumors, including several that are known to have XPG mutations. If the hypothesis is correct that XPG, newly identified as an important HRR protein, plays a role in the initiating events in HGS-OvCa, these results have the potential to provide a new marker for ovarian cancer susceptibility, new therapeutic strategies, and improved understanding of mechanisms involved in ovarian carcinogenesis.